index.rst

Checking of type hints using mypy

Optional type hints can be added to python code which allows specification of the type of a variable. Type hints were specified in PEP484.

Type hints are ignored when running the code but can be statically analysed using a various tools:

• Mypy.
• Pyright

Mypy is used for Nashpy.

For example, consider the file main.py:

/_static/contributing/discussion/mypy/main.py

After installing mypy:

$ python -m pip install mypy

If we check the annotations present in the file:

$ python -m mypy main.py
Success: no issues found in 1 source file

There are no issues because there are no annotations. If the following annotations are added:

/_static/contributing/discussion/mypy/main_with_wrong_types.py

We get:

$ python -m mypy main.py
main_with_wrong_types.py:17: error: Argument 1 to "len" has incompatible type "Iterable[Any]"; expected "Sized"
Found 1 error in 1 file (checked 1 source file)

Mypy has found an error here: the Iterable type does not necessarily have a length. The following modifies this:

/_static/contributing/discussion/mypy/main_with_correct_types.py

We get:

$ python -m mypy main.py
Success: no issues found in 1 source file

--ignore-missing-import

In some cases some imported modules cannot be used checked with Mypy, these can be ignored by running the following:

$ python -m mypy --ignore-missing-import main.py

Overlap of functionality with darglint

The python library darglint checks the format of the docstrings. This will also use any type annotations and so the type annotations and the types specified in the docstrings must correspond.